fix(cache): honest settings card — clean threshold, live stats, no-embedder warning

Full audit of the semantic cache against a live instance (mock embedder,
identical requests): the mechanism works — miss at 3.1s, then hits at 2ms
with entries/hits/saved-$ counting. Two honest gaps fixed:

- the threshold round-tripped through f32 and rendered as
  0.9200000166893005 in both the TOML and the settings input — the config
  field is f64 now (writes exactly what you typed) and the input rounds
  legacy values to 2 decimals;
- the card now shows proof of life (entries / hits / hit-rate / saved $)
  and, when the cache is enabled with NO embedding model available, a
  warning that it is silently doing nothing and how to fix it — previously
  that state looked identical to a working cache.

Also audited in the same pass, no changes needed: Prometheus /metrics
(toggle on/off applies live, 404 when off), and all 7 protocol switches
(each gates its endpoint instantly both ways). 4 strings ×7 languages.

web/js/i18n/zh.js

@@ -635,4 +635,8 @@ export default {
   "tier.medium": "中等",
   "tier.high": "困难",
   "bm.scoreHint": "路由器的连续复杂度得分（0..1）— 路由页面上的边界滑块将其划分为层级。",
+  "settings.cache.noEmbed": "缓存已启用，但没有可用的 embedding 模型 — 它没有任何作用。请向模型池添加 kind = \"embedding\" 的模型（如 OpenAI text-embedding），或在下方选择。",
+  "settings.cache.entries": "条目",
+  "settings.cache.hits": "命中",
+  "settings.cache.saved": "已节省",
 };